Sept 18 (Reuters) - Wildlife managers in Montana are trying to pinpoint whether a disease, environmental toxin or chemical agent has caused a die-off of more than 100 whitetail deer in wetlands along a river corridor in the western part of the state.

The Montana Fish Wildlife and Parks office in Missoula began to receive reports from landowners and boaters 10 days ago of dead deer along the Clark Fork River, and state wildlife biologists had tallied 103 deer carcasses by Tuesday.

"The deer appear to drop dead in their tracks," said agency educator Vivaca Crowser, adding that the deer showed no outward signs of injury or sickness.

Wildlife experts said a viral disease transmitted by tiny biting flies that hatch near bodies of water may be the culprit, but they were awaiting results from testing by a state lab of organ and blood samples.

The malady, epizootic hemorrhagic disease, mostly affects whitetail deer and is often fatal to them, causing hemorrhaging of the heart, liver, spleen and all other organs. The disease has not been shown to affect humans, Crowser said.

But there has been no known prior outbreak of the disease in Missoula or elsewhere in Montana west of the continental divide, Crowser said.

"Everything is still on the table in terms of possible causes. But if it's epizootic hemorrhagic disease, that would be unique," she said, adding that the lab was also investigating whether the cause could be an unidentified "toxin, a poison or another disease."

Outbreaks of the epizootic disease in whitetail deer were first documented in 1955 in New Jersey and Michigan. Michigan saw the disease reach epidemic proportions last year, killing nearly 14,900 whitetail deer, according to the state Department of Natural Resources.

There is no effective treatment for the disease, which tends to abate in colder climates after the first hard frost in fall or early winter. (Editing by Cynthia Johnston and Ken Wills)